In the production of film or sheet made of a melt of thermoplastic material it is important that the operation of all components of the system operate as much as possible free of faults. Particularly the extrusion section of the system should work independently of system faults outside the extrusion station because stopping the extrusion due to faults in other stations of the system can cause a substantial economic loss of melt and equipment clogging. Further, following a shut-down of the extrusion process, experience shows that a relatively long time is needed following the resumption of the extrusion, until stable thermal and rheologic conditions are reestablished.
Conventionally, it is possible to stop the extrusion process when an irregularity occurs in the work stations downstream of the extrusion station, for example when the extruded melt film is not continuously and uniformly withdrawn from a cooling roller onto which the film is pulled out of the extruder. Such irregularity in the further treatment of the film downstream of the extruder may have many causes. For example, the film can rip in a section the further treatment station downstream of the extruder. Such rips may occur in the zone of the longitudinal stretching or in the cross-stretching zone as well as in the zone where the film is transferred from the last stretching station to a take-up roller. Another problem can, for example, be caused when a film is wound onto the surface of any one of the rollers in the system from which a previous film portion has not been properly removed. Other problems can be caused by a defect in any one of the components of the film production system. Any one of the faults mentioned as examples and other faults cause the operator to stop the extrusion any time when it is no longer possible to maintain a continuous uniform withdrawal or take-up of the melt film by maintaining a uniform take-up or pulling force on the melt film. Stopping the extrusion involves switching off the extruder altogether, which is not desirable for the reasons stated above.
The cooling roller is a critical component in such a film production system because the film as it is withdrawn from the wide slot of the extruder nozzle or sheet die, must loop uniformly around the cooling roller and it must be uniformly withdrawn from the cooling roller to avoid forming wrinkles. As long as the uniform withdrawal of the extruded melt film continues at an adequate withdrawal speed, the extrusion process can continue as well. Under certain circumstances it is even possible to reduce the throughput through the extruder, namely to reduce the quantity of the produced extrudate by slowing down the withdrawal. However, as soon as the required pulling force is no longer uniformly applied to the film as it exits from the extruder nozzle or sheet die, the film will no longer be properly released from the cooling roller, whereby the melt film will be rolled up on the cooling roller which is positively driven. As a result, the extrusion process and the drive of the cooling roller must be stopped.
German Patent Publication DE 3,635,302 C2 (Dietz et al.), published on Apr. 28, 1988, discloses a method and an apparatus for cooling a melt film extruded through a sheet die into a cooling system. The aim of this prior art is to rapidly cool down the melt film downstream of the extruder to low, refrigerating temperatures without causing the melt film to pop off any metal surface of the cooling system, particularly the cooling roller. The problem of a non-uniform withdrawal of the melt film from the nozzle is not addressed by the just mentioned German Patent Publication.
German Patent DE 3,907,036 C1(Wetzel et al.), published on Sept. 13, 1990, discloses a cooling device for thermoplastic synthetic film material. Two rows of guide rollers are mounted in an elongated cooling trough. The spacing between the cooling roller rows can be increased to provide more film storage or decreased to reduce the film storage for film passing from an extruder through the cooling trough.
German patent DE 4,113,746 C1(Puetz et al.), published on Apr. 9, 1992, discloses a winding roller for film or sheet material coming from an extruder. As the diameter of the winding roller increases the pulling force changes. However, it is desirable to withdraw the film or sheet material with a uniform pulling force. For this purpose an electric motor driving the winding roller, is controlled to make sure that the exerted torque takes the increasing winding roller diameter into account.
U.S. Pat. No. 4,609,336 (Stevenson et al.), issued on Sept. 2, 1986, discloses a sheet or film take-up system with a floating roller referred to as "accumulator" positioned between the extrusion die and a take-away conveyor. So-called LVDTs (linear variable differential transformers) are used to measure or sense the thickness of the extruded film which must be kept uniform. The position of the accumulator is controlled for compensating surges in the extrusion.
The above discussed prior art leaves room for improvement with regard to the continued extrusion even if faults should be detected in any of the system stations downstream of the extruder as viewed in the feed advance direction of the film.